Multistage vacuum tube amplifier



April 23, 1940. w. VAN B. ROBERTS MULTISTAGE VACUUM TUBE AMPLIFIER FiledMay 27, 1938 GLASS ENI/FL OPE i m-MI L7 INVENTOR WAL rm VAN B. ROBERTSBY ATTORNEY Patented Apr. 23, 1940 PATENT OFFICE H l MULTISTAGE VACUUMTUBE AIVIPLIFIER Walter-van B. Roberts, Princeton, N. J assignor toRadio Corporation of America, a corporation of Delaware App ication May27, issaserial No. 210,384

2 Claims. (or. 250-415) My present invention relates to multi-stagevacuum tube amplifiers generally, and especially to portablebattery-operated amplifiers.

Conventional amplifiers as used in the past for portable purposes havesuffered the drawback that/relatively heavy and expensive batteries havebeen required. Furthermore, two distinct batteries have customarily beenemployed, one for the filaments of the tubes and another battery ofrelatively high voltage for the plate circuits.

Usually one of these batteries runs down more rapidly than the other andthe user may not know which battery requires replacement. Furthermore,it is necessary for him to buy two difierent styles of battery and thehigh voltage battery is not available at all stores and is ratherexpensive.

It is, therefore, the object of the present invention to provide a.multi-stage vacuum tube ampli- 2U fier capable of operation on a singlebattery composed of individual cells which are widely distributed at avery low price for flashlight purposes. A further object is to providesuch an amplifier capable of operation on a relatively small number ofsuch cells, for example or of them.

If it is attempted to utilize the filamentary cathode type of tubes on asingle battery with uniform drain of current throughout the bat- 30 teryby connecting the filaments in series across the whole battery, thedifiiculty arises that the potential difference available between thepositive terminal of the battery and the cathodes of the various tubesis different for the different tubes. Furthermore, if a number ofseparate tubes-are used, there is inevitably a considerable heat loss atthe ends of the filaments of each tube. In accordance with theinvention, therefore, a plurality of tubes is constructed having a wcommon cathode of the heater type and a reflect ing enclosure about theentire structure for reducing radiation heat losses. With thisarrangement cathode heating may be obtained from a battery which is alsoused for the plate circuit energization of the various stages, and endlosses are reduced to a minimum by the use of the single cathode, whileradiation losses are also reduced by the reflecting enclosure so thatthe power drain for cathode-heating purposes is 0 aminimum.

The accompanying drawing represents a typical amplifier systemconstructed in accordance j with the invention. A two stage amplifierarrangement is shown for simplicity, but it will 5 be understood thatmore stages may be employed if desired. In the drawing, referencecharacter 5 denotes a source of signals to be amplified, such as adynamic type microphone. The ground symbol in. the drawing represents acommon point rather than an actual ground, inasmuch as the 5 amplifieris particularly intended for portable use such as, for example, indeai-aid sets. A grid electrode 2 is wound around a cathode 3 which isheated by a heater wire 3 passing through an insulating tube 5. The tubestructure indicated in the drawing is a sectional view taken through theaxis of the tube. One end of heater wire 4 is brought out through aglass envelope 6 to the positive terminal of battery 1 while the otherend of heater wire 4 is connected to cathode 3 and thence, by Way ofWire 8 and resistance 9, to the negative terminal of the battery.

Resistance 9 is shunted by condenser, Ill and provides a positive biasfor the cathode with respect to the ground in a well known manner.Signals impressed upon grid 2 causes variation of space current betweencathode 3 and plate l l the latter plate being connected through theprimary winding of transformer [2 to the positive terminal of battery 7.Voltage developed across the secondary of transformer 12 is impressedupon the grid 2 of a second grid-plate assembly similar to thatpreviously described, and the output plate I! of the second tube sectionis utilized in telephones l 3. It is not believed necessary to describethe operation of the second tube section in further detail, as itsoperation is quite conventional.

In order to reduce radiation losses from cathode 3 to a minimum ametallic, reflecting enclosure or shell l4 surrounds the entireelectronic system. Enclosure It contains a metallic apertured disk itwhich acts as an electrostatic shield between adjacent tube sections.Further disks l t of mica act as spacers for centering the cathode 3within the grid and plate structures which are supported by glass beadsH where the leads from said structures pass through the reflectingenclosure Hi. A second reflecting metallic enclosure or shell It maysurround the first mentioned structure M if desired, both of thestructures l4 and it being attached to an insulating disk It throughwhich insulating tube 5 projects. Cathode connection wire 8 may beconductively connected to both of the reflecting enclosures l4 and it,but the grid and plate leads must, of course, pass through theseenclosures without conductive connection thereto. The grid and plateleads and the cathode and heater leads are brought out through glassenvelope 6 by way of the usual seals.

Cathode 3 is coated on its outer surface with electron emissive materialcapable of producing electron emission at a much lower temperature thanelectron emission occurs from the grid wires, so that grid emission willbe negligible even if the grid temperature should arise to thermalequilibrium with the radiation temperature in the space between cathode3 and reflector l4. While reflector M reduces the radiation losses,nevertheless the actual temperature of M will rise to a rather highvalue, so that it would cause some radiation losses from its outersurface. Such losses are further reduced, however, by the secondenclosure I8, and, in order to prevent thermal conduction betweenenclosures l4 and I8, they are not connected metallically together atany point other than possibly wire 8 whose thermal conductance is small,due to its small cross section.

The grid and plate elements are preferably spaced as close as possibleto cathode 3, in order to permit satisfactory operation at relativelylow plate voltage, but, if desired, such operation may be obtained byinterposing a space charge grid in well known fashion between thecathode and the control grids. It is also a matter of individual choicewhether screen grids be provided between the control grids and plate ofany tube sections.

While I have indicated and described a system for carrying my inventioninto efiect, it will be apparent toone skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is: y

1. A space discharge tube including a hot electron emission element, anelectrode mounted to receive electrons therefrom, a reflection shellsurrounding the emission element and said electrode, a second reflectionshell surrounding the first shell and spaced therefrom, and a conductorconnecting the shells and emission element solely at single pointsthereof.

2. An electron discharge device comprising an envelope, a cathode, agrid and an anode assembled in a unitary electrode assembly, a heatreflecting shell enclosing said electrode assembly, a spacer of goodheat insulating material, said electrode assembly being mounted on saidspacer and said shell being supported on said spacer, said envelopeenclosing said shell and thermally insul'ated therefrom.

WALTER VAN B. ROBERTS.

